Variable condenser



Patented Oct. 24, 1933 PATENT OFFICE VARIABLE CONDENSER Georges EmileMarie assignor,

Pontonnier, by mesne assignments,

Paris, France, to Radio Corporation of America, New York, N. I, acorporation of Delaware Application February s, 1928, Serial No.

and in France February 15, 1927 12 Claims. (Cl. 250-40) My -inventionrelates to electrical tuning systems, and more particularly to condenserarrangements utilized in electrically tuned circuits.

Variable condensers of the type known as straight line frequencycondensers intended for electrical circuit tuning, are frequentlyemployed, at present, more particularly in receiving sets for wirelesstelegraphy and telephony. The profile, or cross section, of the bladesof such a variable condenser, or of the cams or other equivalent drivingdevices of the rotors of the variable condenser, is easily calculated,if the ratio between the maximum and minimum capacities of the tuningcircuit containing the said condenser is known.

- When the ratio between the maximum and minimum capacities of thetuning circuit in which the variable condenser having its bladesdesigned according to the desired profile or cross section, is changed,then it is necessary to alter the design of the profile or crosssection, it being understood that the maximum and minimum capacities ofthe tuning circuit may be obtained when the condenser plate sections arefully overlapped and fully withdrawn from each other respectively. Inother words, given an electrical tuning circuit in a receiving setcomprising an inductance of a fixed value and a variable condenserprovided with blades having a predetermined profile especially designedto obtain straight line frequency over a given range of frequencies, itis necessary, in the prior art, to change the said profile or crosssection of the blades, if it is desired to employ the said variablecondenser for tuning through a different range of frequencies and retainthe straight line frequency characteristic.

Now, I have devised a method of, and means for remedying theaforementioned disadvantage, and it is thereby possible, according to mypresent invention, to vary the ratio of the maximum and minimumcapacities of the tuning circuit in which a variable condenser,operating over its full range and with a straight line frequencycharacteristic, is employed within fairly large frequency range limits,without materially departing from the initial characteristic of the saidvariable condenser, the means for accomplishing this novel resultembodying capacities added to the circuit in addition to the variabletuning condenser, and added in quantities following a predetermined law.

Accordingly, it is one of the main objects of the present invention toprovide a tuning circuit in a receiving set for wireless telegraphy ortelephony which tuning circuit embodies a tuning element designed toimpart a straight line frequency characteristic to the tuning circuit,and additional means associated with the said tuning circuit for varyingthe ratio of maximum to minimum capacity of the tuning circuit whilesubstantially adhering to the said straight line frequencycharacteristic thrqughout a plurality of frequency ranges withoutaltering the physical design of the said tuning element.

Another important object of the invention, which is broader than thatset forth in the preceding paragraph in not pertaining specifically to astraight line frequency characteristic, is to provide a tuning circuitarrangement for a receiving set, the circuit arrangement comprising atunable variable condenser, an additional capacity of a predeterminedvalue in series with said variable condenser, and a second additionalcapacity of a predetermined value in shunt with said variable condenser,said additional capacities being adapted to vary the ratio of maximum tominimum capacity of the tuning circuit arrangement so as to change thefrequency range to another desired frequency range, without altering thephysical design of the said .variable condensers', while yet preservinga desired characteristic of variation of frequency with change incapacity of the tuning condenser.

Other objects of the invention are to improve generally the simplicityand efliciency of tuning circuits employing straight line frequencycon-' densers, and to provide particularly a tuning circuit for areceiving set which is not only reliable in operation but economicalv toassemble.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organi- 9 zations whereby my inventionmay be carried into effect.

In the drawing, 1

Fig. 1 diagrammatically represents one circuit arrangement including theinvention,

Fig. 2 diagrammatically shows 'a circuit arrangement embodying amodified form of the invention.

Referring now to the accompanying drawing wherein like characters ofreference indicate the same parts in the different figures, Fig. 1 showsthe invention embodied in a conventional receiving circuit adapted forwireless telegraphy or telephony. This circuit embodies a source ofsignal energy 1, which may be any well known type of signal energycollecting means utilized in conventional receiving circuits. As is wellknown to those skilled in the art, such a collecting means, or morebroadly source of signal energy, is coupled, as at M, to a resonant ortuning circuit. While I have shown this coupling M as a transformercoupling, it is of course understood that the energy collecting meansmay be coupled to the tuning circuit in any other well known fashion.

The tuning circuit includes an inductance coil 3 in shunt relation witha variable condenser 1. The output terminals 2, 2 of the tuning circuitare connected to any well known type of translating device, as well asthe terminals of the inductance 3, the same being conventionally shownin the drawing, andthe output circuit of the device may be connected toany well known type of utilization means.

The variable condenser 1 is preferably given such a physical design thatits variation imparts a straight line frequency characteristic to thetuning circuit in which it is disposed. This is easily accomplished, asexplained heretofore, by proper design of the profile, or cross section,of the blades of the condenser, or of the cams or other equivalentdriving devices, and is easily calculated by methods well known to thoseskilled in the art when the ratio between the maximum and minimumcapacities of the tuning circuit is known.

It can easily be seen that in the prior art when the physical design ofthe variable condenser 1 has been determined for a given range offrequencies, the variable condenser can be used for a different range offrequencies, with the same tuning. characteristic and the same value forthe inductance coil 3, only by changing the physical design of thevariable condenser 1. Thus, when the variable condenser 1 has beendesigned for one range of frequencies so as to impart a straight linefrequency characteristic to the tuning circuit, the same variablecondenser 1 can be used for a diiferent range of frequencies and stillpossess a straight line frequency characteristic, only by redesigningthe profile or cross section of the blades of the condenser 1. As hasbeen explained, this involves serious disadvantages in receiving circuitdesign of the prior art.

According to the present invention it is not necessary to alter thephysical design of the variable condenser 1, in order to employ the samecondenser for a different range of frequencies, and yet retain thedesired tuning characteristic.

This is accomplished by connecting, in series with the variablecondenser 1, an additional capacity I which has a value very muchgreater than the minimum capacity of the tuning circuit, and connectingin parallel with the variable condenser 1, an additional capacity 7having a value which is much smaller than the maximum capacity of thetuning circuit and the capacity I.

It will be understood that the values of the additional capacity I and'y depend upon the ranges of frequency for which the tuning circuit isto be used. Thus, while for one given range of frequencies the tuningcircuit including the inductance 3 and the variable capacity 1 has astraight line frequency characteristic, for another range of frequenciesit being understood that the physical design of the variable condenser 1is not altered, the additional capacity I and the additional capacity 7are connected in series and in shunt respectively with the variablecondenser 1, and given values (according to a predetermined cluding thesame physical design of the variable I condenser 1 for still anotherrange of frequencies. it is only necessary to change the value of theseries additional capacity I and the shunt additional capacity 7according to the aforementioned predetermined law. Of course, while Ihave shown these additional capacities as fixed capacities it is to beclearly understood that their values are adjusted when changing from onerange of frequencies to another range of frequencies, this beingaccomplished by substituting fixed capacities of different values, or inany other desired manner.

The numerical values to be given to the additional capacity I and theadditional capacity '1 are those that shall satisfy substantially thefollowing equation or relation:

In the lastequation, 0 represents the maximum capacity of the tuningcircuit including the tuning condenser and incidental capacities priorto the addition of the fixed condensers I and 'y, and c is the minimumcapacity of the same circuit prior to the addition of the fixedcondensers I and 'y.

Also, the ratio of the frequencies covered by the whole set ofcondensers, compared to the ratio of the frequencies covered by theinitial condenser is equal to:

It has been found, that the best results are obtained when the variablecondenser 1 is constructed for a ratio of maximum capacity to minimumcapacity which is about 9 or 10 to 1.

In Fig. 2 I have shown a circuit embodying a modified form of theinvention. In this circuit, the additional capacity I still is connectedin series with the variable condenser l, but the additional capacity 7is connected directly in shunt with the variable condenser 1; that is,both the variable condenser 1 and the additional capacity 7 areconnected to the same terminal of the additional capacity I. Here,again, as explained in connection with Fig. 1, the terminals of the coil3 are connected to terminals 2, 2'. The values of the additionalcapacities are changed depending upon the rangeof frequencies to whichthe tuning circuit including the inductance coil 3' and the variablecondenser 1 is to be employed.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications in the circuitarrangements, as well as in the apparatus employed, may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be performed, I declare thatwhat I claim is:--

inductance coil, a variable condenser in shunt relation with said coil,the aforegoing circuit which is small compared to C manor? having amaximum capacity C and a minimum capacity 0, said condenser beingadapted to tune the coil between frequency limits f1 and In according toa predetermined Q law, a capacity I which is large compared to c inseries with said variable condenser, and a capacity 1 in shunt relationwith the latter, whereby the variable condenser is adapted to tune theresulting circuit between diiferent frequency limits F: and F4,substantially according to the aforesaid law and without alter.- ing thephysical design of the variable condenser, the relation between all theaforementioned capacities being defined by the equation:

( ea er- 2. In combination, in an oscillation circuit, an inductancecoil, a variable condenser in shunt relation with said coil, theaforegoing circuit having a predetermined ratio between its maximumcapacity C and its minimum capacity 0, and additional means in saidcircuit, said means comprising a capacity I which is large compared to cin series with the variable condenser, a capacity 7 which is smallcompared to C in shunt relation with the latter, the equation definingthe relations between all of said capacities.

3. In combination, in an oscillation circuit, an inductance coil, avariable condenser in shunt relation with said coil, the aforegoingcircuit having a maximum capacity C and a minimum capacity 0, saidcondenser being adapted to tune the coil between frequency limits I; andf: according to a predetermined law, a capacityl in series with saidvariable condenser, and a capacity 7 in shunt relation with the latter,whereby the variable condenser is adapted to tune'the resultant circuitbetween frequency limits I1+Fa and f:+F4, where F: and F4 arepredetermined frequency increments, substantially according to theaforesaid law and without altering the physical design of the variablecondenser, the relation between all the aforementioned capacities beingdefined by the equation:

the capacity 7 being small compared to C, the capacity I being greatcompared to the capacity c.

4. An oscillation circuit comprising an inductance coil, a tuningcondenser in shunt relation with said coil, said aforegoing circuithaving a maximum capacity C and a'minimum capacity c, means foradjusting the capacity of said condenser to tune the coil according to apredetermined law, and a capacity network in said circuit including afixed condenser 01' capacity 1 in series with the tuning condenser,which is large compared to the capacity c, a fixed condenser of capacity7, in shunt relation with the tuning condenser, which is small comparedto the capacity C, the said capacities being chosen according to theequation:

C l r)( E) said network enabling the tuning condenser to tune theresultant circuit according to the said law, for all values of I and 'ysatisfying their aforementioned relations.

5. An oscillation circuit comprising an inductance coil, a tuningcondenser in shunt relation therewith, said aforegoingmcircuit having amaximum capacity C and a minimum capacity 0, a capacity network in saidcircuit including a fixed condenser of capacity I in series with thetuning condenser and being large compared to capacity c, a fixedcondenser of capacity '7, in shunt relation with the tuning condenserand being small compared to capacity C, means for adjusting the capacityof said tuning condenser, said fixed condensers being chosen to satisfythe relation whereby the ratio of the rates of change, at two antcircuit capacity, with respect to displacement of said adjusting means,is the same as the ratio of the rates of change, at the same two valuesof displacement of said adjusting means, of the square root of thereciprocal of the capacity of the variable condenser, with'respect todisplacement of said adjusting means.

6. An oscillation circuit qmprising an inductance coil, a tuning,condenser in shunt relation with said coil, means for adjusting thecapacity of said condenser to tune said coil over a range of frequencieshaving a given ratio of limiting frequencies, said tuning condenserbeing operable to give a predetermined relation between frequencyvariation and displacement of said adjusting means, said aforegoingcircuit having a maximum capacity C and a minimum capacity 0, andadditional means in said circuit whereby said condenser is adapted totune said circuit over a range of frequencies having a diiferent ratioof limiting frequencies according to a relation between frequencyvariation and adjusting means displacement which is related in a desiredmanner to said first relation, said additional means 7. In combination,in an oscillation circuit, an inductance coil, a variable condenser inshunt relation with said coil, the aforegoing circuit having a maximumcapacity C and a minimum capacity 0, said condenser being adapted totune the coil between frequency limits f1 and f2 according to astraightline frequency variation law, a capacity I which is largecompared to c in series with said variable condenser,'and a capacity 7which is small compared to C in shunt relation with the latter, wherebythe variable condenser is adapted to tune the resultant circuit betweenfrequency limits f1+F3 and fa+F4, where F3 and F4 are predeterminedfrequency increments, substantially according to the, aforesaid law andwithout altering the physical design of the variable condenser, therelation between all the aforementioned capacities being defined by theequa- 8. In combination, in an oscillation circuit, an inductance coil,a variable condenser in shunt relation with said coil, the aforegoingcircuit having a maximum capacity C and a minimum caadapted to tune theresultant circuit between frequency limits n+3: and fa+F4, where F: andF4 are predetermined frequency increments, substantially according tothe aforesaid law and without altering the physical design 0! thevariable condenser, the relation between all the aforementionedcapacities being defined by the equation: 1

the capacity 7 being small compared to C, the

capacity I being great compared to the capacity c.

9. In combination, in an oscillation circuit, an inductance coil and avariable tuning condenser, means for increasing the frequency to whichthe oscillation circuit is tuned when the variable condenser is adjustedto its maximum capacity position, said means comprising a fixed capacityin series with said variable condenser, and a second means fordecreasing the frequency to which said oscillation circuit is tuned whenthe variable condenser is adjusted to its minimum capacity position,said second means comprising a fixed capacity in shunt relation to saidvariable condenser, said series capacity being large compared to theminimum capacity of the circuit consisting of the variable condenser andthe coil, and said shunt capacity being small compared to the maximumcapacity of the last named circuit.

10. A capacity network including a variable condenser, a fixed capacityin series therewith, so chosen that the maximum capacity of the networkis made less than the maximum capacity of the variable condenser, and afixed capacity in shunt relation with said variable condenser so chosenthat minimum capacity of the network is made greater than the minimumcapacity of said variable condenser, said series capacity being largecompared to said network minimum capacity, and said shunt capacity beingsmall compared to said network maximum capacity.

11. In combination with an oscillation circuit consisting of a coil anda variable condenser which is adapted to vary the resonant frequency fof the circuit over a range of frequencies, said above oscillationcircuit having a maximum capacity C and a minimum capacity c, anauxiliary condenser I in series with the variable condenser having acapacity large compared to c, and a second auxiliary condenser 7 inshunt with the variable condenser having a capacity small compared to C,said auxiliary condensers being chosen to satisfy the relation sultantcircuit varies in such a way that df is substantially constant betweenpredetermined limits of variation of the variable condenser.

12. In combination with an oscillation circuit comprising an inductancecoil and a variable tuning condenser in shunt relation with said coiland adapted in the absence of other reactances to tune the circuit overa range of frequencies in accordance with a predetermined frequencycurve, means in the aforegoing circuit including condensers of suitablecapacity value in series and shunt respectively with the tuningcondenser for enabling the tuning condenser to tune the resultantcircuit in accordance with a different frequency curve bearing asubstantially linear relation with the first mentioned curve.

GEORGES EMILE MARIE PONTONNW.

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